'Reminder' quake shakes up Davis County

Although Wednesday night’s earthquake wasn’t particularly impressive, seismically speaking, it sent a shock wave — both literally and figuratively — throughout northern Utah.

The 3.3-magnitude earthquake — which occurred at 10:34 p.m. and was centered in the mountains 10 miles east of Bountiful — was just a “small” quake, according to the University of Utah Seismograph Stations in Salt Lake City. But it ended up generating some big talk in northern Utah communities, including on social media sites.

“With a magnitude 3.3 like this, this is sort of at the lower threshold of what people can feel,” said Katherine Whidden, a research seismologist with the University of Utah.

Whidden says earthquakes of that magnitude — 3.0 to 3.9 — occur about eight times a year in Utah.

“So they’re not uncommon,” she said.

But the exciting part, according to Whidden, is that so many people felt it.

“These sorts of earthquakes do happen all the time in Utah,” she said. “What’s unusual about this one is that it was near the most populated area in the state, so a lot of people felt it.”

Reports to the “Did You Feel It?” page on the U.S. Geological Survey website show that the Utah quake was felt as far away as North Ogden, Tooele, Provo and Park City. By midday Thursday, nearly 3,000 people had reported their earthquake experience to the web page. No damaged was reported from the quake.

Whidden said this relatively minor earthquake generated a degree of excitement among her colleagues, because many of them felt it.

“Several people here, it was the first earthquake they’ve ever felt,” Whidden said. “And well, you know, I’ve never felt an earthquake, and I didn’t feel this one last night. I’m so mad.”

Many northern Utahns reported feeling two quick jolts on Wednesday night, and Whidden says that was most likely what’s called the P-wave, followed by the S-wave (short for “primary wave” and “secondary wave”). Those two waves come at the beginning of an earthquake.

“Looking at the data, two larger waves came in first, then a bunch of little stuff as waves bounced around in the earth, kind of dying off,” Whidden explained. “Think of it like throwing a rock in a pond, with the bigger waves followed by the smaller ones.”

According to seismic instruments, the quake was centered about seven miles deep and lasted about 20 seconds in all, with the shock waves traveling out from the epicenter at about 4 miles per second.

So, is Wednesday’s quake a precursor to something bigger? Probably not, says Whidden, although there is a small possibility.

“Any time there’s an earthquake, it does slightly increase the chance it’s a foreshock, and that a bigger one is coming,” she said. “Here in Utah, there’s a one-in-20 chance — about 5 percent — that a smaller earthquake will lead to a bigger one.”

But not “The” Big One, the massive 7.0-magnitude earthquake predicted sometime in Utah’s future for the Wasatch Fault.

Whidden said earthquake fault lines are mapped all over Utah, but Wednesday’s quake did not occur along any known fault. But then again, few ever do.

“It’s probably not a ‘new’ fault, but it’s not on a fault we knew of,” she said. “But then, all of the earthquakes that we record are almost never on a fault line that we know about.”

Nor did the quake occur in a particularly seismically active area. In a search of 50 kilometers (about 31 miles) surrounding the site of Wednesday’s quake, Whidden found a 5.2-magnitude earthquake back in 1962, and a handful of magnitude-3 quakes going back to the 1960s.

For all the commotion Wednesday’s temblor caused in the community, the epicenter itself seems fairly insignificant. Data suggests the fault area involved was just a tenth of a mile squared, which works out to a little over 500 square feet — slightly larger than the average hotel room.

What’s more, the fault moved only ever-so-slightly in Wednesday’s quake.

“The amount it slipped would be in millimeters,” Whidden said.

By contrast, when The Big One finally does hit Northern Utah, Whidden says the fault could slip between five and 10 feet.

“Imagine a five- to 10-foot cliff appearing in the middle of the city all of sudden,” she said.

Whidden said Wednesday’s shake was a gentle reminder.

“I always like to think of these as a reminder that we can have a much larger earthquake,” she said. “We live in earthquake country, and we all need to be prepared with a 72-hour kit and an emergency plan.”

Contact Mark Saal at 801-625-4272 or msaal@standard.net. Follow him on Twitter at @Saalman. Like him on Facebook at facebook.com/SEMarkSaal.